Rickettsia rickettsii is the causative agent of one of the most severe tick-borne diseases in the United States. The compounding factors of rickettsial pathogenicity and route of tick transmission (horizontal and vertical) influence the ecology and epidemiology of human rickettsioses. Our long term goal is to elucidate the molecular mechanisms that are central to rickettsial transmission by ticks. The bacterial-derived molecular determinants of rickettsial infection of vertebrate cells have been described using various in vitr models. Our research indicates that rickettsial infection of tick hosts also utilizes similar processes described for infection of vertebrate host cells. However, the role of rickettsial proteins in tick infection is unknown. Likewise, although the tick actively responds to the infecting rickettsiae, the central mechanisms orchestrating the tick-derived response are not known. Thus, the experimental focus of this application emphasizes the roles of pathogenic determinants underlying rickettsial infection and the bacterial-induced alterations of the tick immune response mediating successful rickettsial colonization of the vector. The following specific aims will provide a better understanding of the constituents of rickettsial infection and transmission in arthropod vectors. 1) Defining the contribution of conserved surface cell antigens (Sca) to SFG Rickettsia infection of the tick vector. Sca proteins are known to be essential to rickettsial infection and dissemination in vertebrate cells. We hypothesize that there will be a distinct profile associated with infection of the arthropod vector. The objective of this aim is to define the role for rickettsial proteins during the infection process in the tick host. 2 To determine how the tick immune response to SFG Rickettsia correlates to vector competence. A vigorous tick response to SFG Rickettsia supports a species-specific host response to rickettsial infection. Therefore we hypothesize that a central molecule, Relish, is responsible for the balance between successful rickettsial colonization and clearance from the arthropod host. The objective of this aim is to delineate the IMD immune signaling system focusing on the production of active Relish in the tick host.